The technological background is illustrated by Patents FR-A-1,433,702 and WO-A-96 20259.
The steam-cracking process is the basic process of the petrochemical industry and consists in cracking a feedstock of hydrocarbons and water vapor at high temperature and then abruptly cooling it. The main operating problem arises from the deposition of carbon-containing products on the inner walls of the unit. These deposits, which consist of coke or heavy pyrolysis tars that are condensed and more or less agglomerated, limit heat transfer in the cracking zone (in a pyrolysis pipe coil) and the indirect quenching zone (effluent quenching exchanger), thus requiring frequent shutdowns to decoke the unit.
In patents EP-A-419 643, EP-A-425 633 and EP-A 447 527 there is proposed a process for in-service decoking of steam-cracking units by injecting erosive solid particles in order to solve coking problems and to obtain continuous or approximately continuous steam cracking (for example, cycle periods on the order of 1 year).
The erosive solid particles can be injected upstream from the cracking zone of each furnace in order to scrape out the coke that is deposited in the pyrolysis pipes, and then downstream, the coke that is deposited in the effluent quenching exchangers.
The injections are carried out on line, i.e., either, preferably, during the normal operation of the furnace or during times when the hydrocarbon supply is interrupted briefly, whereby the furnace is then flushed with a stream of water vapor and is connected to the downstream sections of the unit (primary furnace, compression of cracked gases, etc.) This passage under vapor, in the absence of oxygen, can also be used for vapor decoking of the pipes of the furnace when it is carried out over longer periods of time. It is also possible to inject particles during air decoking periods, as air or air/vapor mixtures are being circulated in the unit.
To provide flexible steam cracking that is compatible with the use of heavy feedstocks (gas oil, distillate under vacuum) or olefinic feedstocks in an existing steam-cracking unit that is provided for cracking naphtha, it was found that it is essential to scrape out the coke that was deposited in the effluent quenching exchangers well and that in-service decoking of these quenching exchangers made it possible, unexpectedly, to make an existing steam-cracking unit compatible with a very wide variety of feedstocks and operating conditions. It was also found, unexpectedly, that the coke that was deposited in the quenching exchangers was much easier to eliminate by erosion than the coke in the pyrolysis pipes and that the previously proposed complete process for decoking of the unit by fine erosive particles was very difficult to implement in a reliable manner with flexible operation under inevitably variable conditions: the geometry of the pyrolysis pipes cannot in fact be suitable for all of the feedstocks to ensure correspondence between the erosive local intensity and the local coking speed (whereby the nature of the coke and its hardness can furthermore vary very greatly from one feedstock to the next); in contrast, with flexible operation, i.e., with variable operating conditions, type of feedstock and degree of dilution, the loss of load and the skin temperature of the pipes are no longer reliable indicators of the coking state of a bundle of pyrolysis pipes, and said coking state therefore cannot be known and monitored in real time.